Fluid Dynamics: See Spot Shaking His Coat Dry

When a dog shakes itself off, most people see muddy droplets befouling their nice new couch. Researchers at the Georgia Institute of Technology see complex fluid dynamics at work. “There are so many variables to consider,” says Andrew Dickerson, a grad student in mechanical engineering. “Hair density, size, texture, oiliness, stiffness variability.” To crack the code of “dynamic water repellency,” the team armed itself with a $27,500 camera that can capture 1,200 frames per second, then spent 240 hours filming and analyzing shake footage of 27 drenched animals. They soaked nine dogs in their lab and several animals at the Atlanta zoo, including a 574-pound brown bear, with a garden hose. For rats and mice, they used spray bottles. So how will this watershed in scientific achievement benefit humankind? Modeling the dispersion of droplets could help improve washing machines, painting equipment, and even the spin coaters used to apply thin films to silicon wafers. The researchers also cite the poor Mars rover, which lost power due to its inability to shed accumulated dust. Maybe future space bots will be able to wriggle free of Martian dirt like a spaniel at the beach.

When It Comes to Shedding Water, Size Matters

Researchers found that animals’ oscillations per second tended to decrease with increasing body size.
A mouse oscillates 29 times per
second, a dog like the one in this
picture manages about 5, and a brown bear clocks in at 4. For animals to dry
themselves, they shake at tuned
frequencies of a power law the team dubbed the “wet-dog-shake rule.”

DV = AR x (2pi x SF)

Formula for calculating drop velocity (DV),
the speed at which water droplets leave the fur. AR is animal radius, and SF is shaking frequency.